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Cr2O3 adopts the corundum structure, consisting of a hexagonal close packed array of oxide anions with 2/3 of the octahedral holes occupied by chromium. Similar to corundum, Cr2O3 is a hard, brittle material (Mohs hardness 8-8.5).[1] It is antiferromagnetic up to 307 K, the Neel temperature.[2][3] It is not readily attacked by acids or bases, although molten alkali gives chromates (salts with the CrO2−
4 anion, not to be confused with the related mineral chromite). It turns brown when heated, but reverts to its dark green color when cooled. It is also hygroscopic.

Cr2O3 occurs naturally in mineral eskolaite, which is found in chromium-rich tremolite skarns, metaquartzites, and chlorite veins. Eskolaite is also a rare component of chondrite meteorites. The mineral is named after Finnish geologist Pentti Eskola. [1]

The Parisians Pannetier and Binet first prepared the transparent hydrated form of Cr2O3 in 1838 via a secret process, sold as a pigment.[4] It is derived from the mineral chromite, (Fe,Mg)Cr2O4. The conversion of chromite to chromia proceeds via Na2Cr2O7, which is reduced with sulfur at high temperatures:[5]

Na2Cr2O7 + S → Na2SO4 + Cr2O3

The oxide is also formed by the decomposition of chromium salts such as chromium nitrate or by the exothermic decomposition of ammonium dichromate.

(NH4)2Cr2O7 → Cr2O3 + N2 + 4 H2O

The reaction has a low ignition temperature of less than 200°C and is frequently used in "volcano" demonstrations.[6]

Because of its considerable stability, chromia is a commonly used pigment and was originally called viridian. It is used in paints, inks, and glasses. It is the colourant in "chrome green" and "institutional green." Chromium(III) oxide is a precursor to the magnetic pigment chromium dioxide, according to the following reaction:[5]

Cr2O3 + 3 CrO3 → 5 CrO2 + O2

It is one of the materials that are used when polishing (also called stropping) the edges of knives, razors, etc. on a piece of leather, balsa, cloth, or other material. In this context it is alternatively known as "green compound".

Chromium(III) oxide is amphoteric. Although insoluble in water, it dissolves in acid to produce hydrated chromium ions, [Cr(H2O)6]3+ which react with base to give salts of [Cr(OH)6]3−.[7] It dissolves in concentrated alkali to yield chromite ions.

When heated with finely divided carbon it can be reduced to chromium metal with release of carbon dioxide. When heated with finely divided aluminium it is reduced to chromium metal and aluminum oxide:

Cr2O3 + 2 Al → 2 Cr + Al2O3

Unlike the classic thermite reaction involving iron oxides, the chromium oxide thermite creates few or no sparks, smoke or sound, but glows brightly. Because of the very high melting point of chromium, chromium thermite casting is impractical.